The present invention relates to an improved process for the preparation of supported chromium oxide-type catalysts used for the polymerization of olefins, and to an improved process for the polymerization and/or copolymerization of ethylene.
The polymerization of olefins, particularly of ethylene, with supported chromium oxide-type catalysts, has been known for a long time. Such polymerization can be carried out as a "solution process", in which the polymer is obtained as a solution in the solvent used for the polymerization, such as is described in U.S. Pat. No. 2,825,721. The polymerization of ethylene using a supported chromium oxide-type of catalyst can also be carried out as a `slurry process`, in which the ethylene polymer is precipitated, and is obtained as a slurry. Such a method is described in U.S. Pat. Nos. 2,825,721 and 3,087,917. Solution polymerization processes are generally carried out at temperatures of at least 120.degree. C., whereas slurry polymerization processes are carried out at temperatures of about 110.degree. C. at the most.
The molecular weight of the resulting ethylene polymer increases as the polymerization temperature is decreased. Thus, with the relatively low temperature slurry polymerization processes, ethylene polymers having very low melt indices, i.e., very high molecular weights, are often obtained. There are many catalyst of the supported chromium oxide-type which are very suitable for use in higher temperature solution polymerization processes operating at temperatures of, for example, 140.degree. C. or higher. However, in slurry polymerization processes, these catalysts yield only high molecular weight polymers.
It is known that the molecular weight of the polymer can be reduced by use of molecular weight controllers. The most common molecular weight controller is hydrogen. However, many supported chromium oxide-catalysts show very little sensitivity to hydrogen. Accordingly, to utilize these catalysts in slurry polymerization processes, large quantities of hydrogen must be added to the monomer in order to obtain the desired molecular weights. Consequently, the effectiveness of the catalyst is strongly reduced, lower yields are obtained and, moreover, large quantities of hydrogen must be recirculated with relatively small quantities of monomer. Such methods are thus economically very unattractive, and are often hardly feasible technically to carry out.
On the other hand, slurry polymerization processes have generally a better efficiency than solution processes. For this reason, there is a need for catalysts which are capable of making lower molecular weight ethylene polymers in slurry polymerization processes, which catalysts are more sensitive to variations in temperature and to molecular weight controllers such as hydrogen. Moreover, there is a wide variation in the correlation between changes in polymerization temperature and resulting polymer molecular weight from catalyst to catalyst. With many catalysts, the molecular weight shows very little variation with changes in the polymerization temperature. In some prior art catalysts, however, it is stronger, and with these catalysts molecular weight can be better influenced by the choice of the polymerization temperature. However, in this respect they are not fully satisfactory, and there remains a need for further improvement. In using these catalysts it is also very desirable that limited quantities of hydrogen can bring about a definite lowering of the molecular weight of the resulting polymer.
Various modifications of supported chromium oxide-types of catalysts have been proposed in order to influence and control molecular weights of ethylene polymers prepared therewith. Thus, it is described in U.K. Pat. No. 1,231,322, and in U.S. Pat. Nos. 3,812,058; 3,862,104; 3,900,457; and 3,974,101, that the melt index of resulting ethylene polymers is a function of the method used to prepare the silica support, and of the activation by heating of the supported chromium oxide. Thus, by making suitable choices among these preparative methods, polyethylene having higher melt indices can be prepared by means of a slurry polymerization process.
It is an object of the present invention to provide an improved process for preparing a supported chromium oxide-type of catalyst which can be effectively utilized in slurry polymerization processes, as well as in solution polymerization processes. It is a further objective of this invention to provide a chromium oxide-type catalyst having greater sensitivity to polymerization temperature and to molecular weight controllers such as hydrogen in affecting the molecular weight of the polymer produced, as compared to known catalysts. It is furthermore an objective of this invention to provide an improved process for the polymerization of ethylene, or the copolymerization of ethylene with one or more .alpha.-alkenes, utilizing the improved catalyst prepared in accordance with this invention.